1. Field of the Invention
The present invention relates to a wire electrical discharge machine configured to be able to apply a variable voltage to a machining gap between a tool electrode and a workpiece and adapted to detect contact according to materials of the tool electrode and workpiece.
2. Description of the Related Art
In a wire electrical discharge machine, contact between a wire electrode and workpiece is detected as follows: a predetermined voltage pulse is applied to a machining gap between the wire electrode and workpiece during non-machining periods, and any change in a gap voltage generated by the predetermined voltage pulse is detected at a contact location between the wire electrode and workpiece.
For example, Japanese Patent Application Laid-Open No. 3-281150 discloses a wire electrical discharge machine which moves a wire electrode relative to a workpiece, compares the gap voltage between the wire electrode and workpiece with a preset reference voltage, and thereby detects any contact between the wire electrode and workpiece. FIG. 5 is a circuit diagram for illustrating a voltage application unit 3 and contact determination unit 4 in the wire electrical discharge machine using a simplified example of detecting contact between a workpiece and wire electrode. In FIG. 5, reference numeral 11 denotes a direct current voltage source in the voltage application unit 3 and reference numeral 13 denotes a direct current voltage source in the contact determination unit 4. Also, reference numeral 12 denotes a switch and 14 denotes a comparator. During contact detection, the switch 12 is closed and a direct current voltage V1 from the direct current voltage source 11 is applied to a machining gap between a wire electrode 1 and workpiece 2.
Also, Japanese Patent Application Laid-Open No. 2002-154015 discloses a wire electrical discharge machine which repeatedly applies an asymmetric bipolar pulse voltage to a machining gap between a wire electrode and workpiece, and detects any contact between the wire electrode and workpiece based on changes in the asymmetric bipolar pulse voltage.
The detection of contact between the wire electrode and workpiece described above involves applying a voltage of a few volts between the electrodes, monitoring the voltage applied between the electrodes, comparing the voltage with a reference voltage, and determining that the circuit is open if the voltage is higher than the reference voltage. On the other hand, if the voltage is lower than the reference voltage, a numerical controller detects a contact location between the wire electrode and workpiece, regarding that the wire electrode and workpiece are short circuited. Normally a voltage pulse with a small voltage value has to be used for the detection because a voltage pulse with a large voltage value will cause great damage to the workpiece due to electrical discharges during detection of an end face of the workpiece, resulting in flaws.
However, when a small voltage value is used, there is a problem in that the end face of the workpiece cannot be detected well because detection voltage does not change due to high resistance between the wire electrode and workpiece end face if the workpiece is made of a low-conductivity material or if the end face of the workpiece is rough due to rust or electrolysis.